This makes it possible to generate packages that you do not have a
local checkout for, e.g. running:
`gomod2nix generate --outdir example/ golang.org/x/tools/cmd/stringer`
This will be useful for packaging dependencies that you are not
developing, but just simply packaging.
This calculates the output hashes in pure Go and removes Nix from
being a run time dependency of gomod2nix.
Additionally it should be faster, but that's untested.